The horizontal directional drilling (HDD) industry traditionally uses walk-over tracking techniques to follow the progress of a bore or utility installation, to find the surface location immediately above a drill bit or backreamer, and to determine the depth of the drill bit or backreamer from that surface location. The primary tracking tools are a subsurface transmitter and a hand-carried surface receiver. The transmitter, located in or very near a boring tool or backreamer, generally emits a magnetic dipole field created by a single coil dipole antenna. The transmitted dipole field can be used to find the surface location over the transmitter, to determine the depth of the transmitter, and to receive data transmitted by the subsurface transmitter when the emitted magnetic field is suitably modulated to convey data.
While walkover tracking is the directional drilling industry's primary tracking technique, the HDD industry is acutely aware of significant limitations of walk-over technology. Walkover tracking requires a minimum crew of two (one machine operator and one tracker operator), and walkover tracking is sometimes impractical, impossible, or dangerous (as when drilling under river beds or canals, when drilling under buildings or other structures, or when drilling under major highways or other thoroughfares with multiple lanes of heavy traffic). In addition, there is a long-felt need for improved drilling accuracy as the bit nears the desired exit point (sometimes called a “bore to the box” operation). Improved navigational information is always an industry need.